The present invention relates to a process for the production of homogeneous, particulate sodium percarbonate (hereinafter abbreviated to "PCS") with active oxygen contents of at least 10% by weight, in particular 13.5 to 14.5% by weight.
Sodium percarbonate is used as the bleaching component in washing, bleaching and cleaning agents in powder form. It is characterized by a high solubility in water and rapid liberation of the hydrogen peroxide and is environment-friendly since its decomposition products do not pollute the environment.
In the literature, the empirical formula Na.sub.2 CO.sub.3.1.5 H.sub.2 O.sub.2 is given for sodium percarbonate with a theoretical active oxygen content of 15.28% by weight. However, in this respect it should not be forgotten that sodium percarbonate produced on an industrial scale from hydrogen peroxide and soda generally is not such a well-defined homogeneous compound but, on the one hand, a mixture of different compounds containing water of hydration with the formulae EQU Na.sub.2 CO.sub.3.1.5 H.sub.2 O.sub.2 EQU Na.sub.2 CO.sub.3.1.5 H.sub.2 O.sub.2.H.sub.2 O EQU Na.sub.2 CO.sub.3.2 H.sub.2 O.sub.2.H.sub.2 O EQU Na.sub.2 CO.sub.3.2 H.sub.2 O.sub.2 EQU Na.sub.2 CO.sub.3.x H.sub.2 O.sub.2
and, on the other hand, depending on the production process, additionally also contains a certain proportion of non-oxidized soda and other production-dependent additives such as sodium sulphate or common salt. The properties of the product are decisively influenced both by the production conditions and the additives concerned not only with respect to the stability but also with regard to e.g. the active oxygen content, the solubility and the bulk density or the grain size of the sodium percarbonate. The active oxygen content thus achievable in industrially produced sodium percarbonate thus amounts to 13.4 to 14.5% by weight only in advantageous cases, but is frequently much lower due to production-dependent additives (sulphate, common salt) and stabilizing measures. The satisfactory solubility of sodium percarbonate, too, is often reduced, e.g. by the production-dependent presence of other salts such as soda, sodium sulphate and common salt. Moreover, the bulk density or grain size of the sodium percarbonate that is achievable in the production processes according to the state of the art is generally only slightly variable and is usually restricted from the beginning to narrow value ranges as a result of the type of process or the soda used.
Increasingly, however, there is a requirement for sodium percarbonates with a high active oxygen content and different bulk densities or grain sizes, in line with the different requirements of the washing agent manufacturers e.g. for use in light powder detergents with a low bulk density or in compact detergents with a high bulk density of the washing, bleaching and cleaning agent components. In this respect it is in particular necessary to match the bulk densities of the individual components of such compositions in order to largely eliminate segregation which would necessarily occur with different bulk densities of the components.
For the production of sodium percarbonate three different techniques are known according to the state of the art: crystallization processes, spray processes and dry processes.
As a rule, sodium percarbonate is produced by the crystallization process. For this purpose, a solution or suspension of soda is reacted with hydrogen peroxide at 10 to 20.degree. C. and crystallized in the presence of stabilizers such as water glass, inorganic or organic phosphonic acids etc. Because of the high solubility of sodium percarbonate, however, it is necessary to salt out the sodium percarbonate from the reaction mixture in order to increase the yield, common salt in a concentration of approximately 240 g/l being preferably introduced into the reaction mixture according to the state of the art. However, the crystallization process is difficult to control so that it is recommended to add the so called crystallization improvers such as polyphosphate or polyacrylate in order to form an advantageous crystal load. The crystallized sodium percarbonate is then removed by centrifuging and dried by the usual processes, e.g. in the fluid bed. However, the PCS obtained by the crystallization process is not the optimum one for many applications.
Spray processes for the production of sodium percarbonate are described in the patent literature e.g. in GB 722 351 and U.S. Pat. Nos. 3,555,696 and 3,801,706. In these processes it is not necessary to filter or centrifuge in order to remove the sodium percarbonate from the mother liquor. Instead, an aqueous solution (or, if necessary, a low concentration suspension) of soda and hydrogen peroxide is dried in a spray dryer in the case of these processes. As a rule, however, spray dried products have a very low bulk density of only approximately 0.35 kg/l and can therefore not be used as such for present day washing agent formulations which increasingly contain granular components with elevated bulk densities. In addition, the spray drying process poses the problem that a strong tendency to caking is observed, particularly when suspensions are used. In some cases, this has a considerable negative effect on the spray process since frequent cleaning of the clogged nozzles is required or spraying of the solution becomes altogether impossible. The spray drying processes of the state of the art consequently preferably operate with solutions. According to GB 722 351 for example, a mixture of soda solution and hydrogen peroxide is passed into the spray dryer before a possible crystallization of PCS begins.
However, when dilute solutions are sprayed, a large quantity of water needs to be removed requiring additional energy expenditure. When an attempt is made to increase the temperature of the feed solution to the spray dryer in order to increase the solubility, increasing decomposition processes are observed in the feed solution, according to U.S. Pat No. 3,555,696. This leads to active oxygen losses and to irregularities in the spray process. To solve this problem, U.S. Pat. No. 3,555,696 suggests that the feed solution to the spray dryer be produced only from stable base compounds (soda) and hydrogen peroxide be introduced into this feed solution immediately before spraying. However, in the case of this process, too, the soda concentration of approximately 20% by wt. remains low and to avoid decomposition processes, low drying temperatures are used which are not particularly suitable in practice (inlet 73.degree. C., outlet 46.degree. C.). In addition, the process is difficult to control.
According to the process of U.S. Pat. No. 3,801,706, a feed mixture for the spray dryer is produced by mixing an aqueous hydrogen peroxide solution of at least 40% by wt. with a soda solution essentially saturated at 20-25.degree. C., which mixture already contains part of the PCS formed in the crystallized form. However, the process is not yet an optimal one since the PCS content of the feed mixture does not exceed approximately 20% by weight.
As a variation of the spray process, solutions of soda and hydrogen peroxide, for example, are continuously sprayed onto a bed of sodium percarbonate fluidized with hot air. The spray and the drying stage can be carried out alternatively in a single or in two stages. According to another variation of the spray process, solutions of sodium carbonate and hydrogen peroxide are sprayed through separate nozzles into a reaction chamber, a hot mixture of air and carbon dioxide being passed simultaneously through the reaction chamber. According to this process, a fairly porous sodium percarbonate is obtained which does not satisfy the requirements for a washing agent composition of present-day standard with respect to its bulk density and attrition resistance.
According to the so-called dry processes, sodium percarbonate is produced by reacting anhydrous or hydrated sodium carbonate (with 75 to 90% by wt. Na.sub.2 CO.sub.3) with a concentrated hydrogen peroxide solution of 50-80% by wt. and evaporating the small quantities of liberated water during the reaction. In the case of this process, an essentially (largely) dry reaction mixture is present throughout the entire reaction. The process can, for example, be carried out in mixers, fluid bed reactors or in tubular reactors with input spray devices for H.sub.2 O.sub.2. Apart from long reaction periods, this process has the disadvantage that no purification of the sodium carbonate produced in this way takes place so that additional measures for stabilizing the product, e.g. an addition of special stabilizers, need to be taken during the reaction. In addition, this process is variable to a limited extent with respect to the granulating properties of the sodium percarbonate, e.g. with regard to the bulk density and the grain size, since the shape of the sodium percarbonate granules correspond essentially (i.e. apart from a slight roundedness attributable to the reaction) to the granular form of the soda used. For the production of sodium percarbonate granules with a high bulk density for compact washing agents, in particular, heavy soda therefore needs to be used which, however, provides little surface area for the reaction with hydrogen peroxide. Consequently, the reaction is incomplete so that only low active oxygen contents and only non-homogeneous products with a high unevenly distributed soda content are obtained whose alkalinity reduces the stability of the product.